U.S. patent application number 13/377651 was filed with the patent office on 2012-04-05 for improved mobility management in a coordinated multipoint network.
Invention is credited to Dirk Gerstenberger, Daniel Larsson, Walter Muller.
Application Number | 20120082058 13/377651 |
Document ID | / |
Family ID | 41818911 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120082058 |
Kind Code |
A1 |
Gerstenberger; Dirk ; et
al. |
April 5, 2012 |
IMPROVED MOBILITY MANAGEMENT IN A COORDINATED MULTIPOINT
NETWORK
Abstract
In one aspect, the present invention in one or more embodiments
improves mobility management--e.g., handover processing
enhancement--by providing/utilizing additional information in
combination with CoMP operation, for enhancing the handover
procedure. For example, the additional information is used to
improve judging the most suitable target cell (14). The additional
information can be, for example, angle of arrival or time advance
of the UE (20) in handover, RTT, power headroom or signal strength.
In at least one embodiment, the information used for handover
processing at source and/or target base stations (12-1, 12-2) is
enhanced or otherwise made richer by the use of "geometric
information" for mobile terminals (20). In one such embodiment, the
geometric information used for handover processing associated with
a given terminal (20) indicates something about the mobile
terminal's position (relative or absolute).
Inventors: |
Gerstenberger; Dirk;
(Stockholm, SE) ; Larsson; Daniel; (Solna, SE)
; Muller; Walter; (Upplands Vasby, SE) |
Family ID: |
41818911 |
Appl. No.: |
13/377651 |
Filed: |
December 28, 2009 |
PCT Filed: |
December 28, 2009 |
PCT NO: |
PCT/SE2009/051505 |
371 Date: |
December 12, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61222769 |
Jul 2, 2009 |
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Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H04W 36/0069 20180801;
H04W 36/0055 20130101; H04W 36/0085 20180801; H04W 36/00837
20180801; H04W 36/08 20130101 |
Class at
Publication: |
370/252 |
International
Class: |
H04W 36/00 20090101
H04W036/00; H04W 24/00 20090101 H04W024/00; H04L 12/26 20060101
H04L012/26 |
Claims
1. A method of mobility management in a coordinated multipoint
network characterized by: sharing geometric information for a
mobile terminal between two or more cells in the coordinated
multipoint network, said geometric information determined from
signal measurements made for the mobile terminal; and determining a
handover target for the mobile terminal from among the two or more
cells, based at least in part on the geometric information.
2. The method of claim 1, further characterized in that said step
of sharing the geometric information comprises a source cell
sending the geometric information to a number of candidate cells
that are prospective handover targets for the mobile terminal, and
further comprising: receiving measurement information from the
candidate cells corresponding to signal measurements made by the
candidate cells based at least in part on the geometric information
sent to them by the source cell; and wherein said step of
determining the handover target comprises identifying the handover
target as a preferred one of the candidate cells, based at least in
part on evaluating the measurement information received from the
candidate cells.
3. The method of claim 1, further characterized in that said
geometric information relates to a relative or absolute position of
the mobile terminal, with respect to one or more base stations in
the coordinated multipoint network.
4. The method of claim 1, further characterized in that said
geometric information includes one or more of: directional
information for the mobile terminal relative to a source base
station; timing advance information for the mobile terminal; and
round-trip-time information for the mobile terminal.
5. The method of claim 1, further characterized in that said step
of sharing the geometric information comprises a source base
station in the coordinated multipoint network sending the geometric
information to a candidate base station in the coordinated
multipoint network that is a candidate for being selected as the
handover target, and further comprising the candidate base station
using the geometric information to improve signal measurements made
by the candidate base station for the mobile terminal, wherein said
signal measurements are evaluated in said step of determining the
handover target.
6. The method of claim 1, further characterized by using the
geometric information in a target base station associated with a
candidate cell in the coordinated multipoint network that is
identified as the handover target, to improve transmissions by the
target base station to the mobile terminal.
7. The method of claim 6, further characterized in that said step
of using the geometric information in the target base station
comprises forming or otherwise steering directional transmissions
from the target base station to the mobile terminal, at least
initially upon the mobile terminal being handed over to the target
base station.
8. The method of claim 1, further characterized by using the
geometric information in a target base station associated with a
candidate cell in the coordinated multipoint network identified as
the handover target, to improve reception at the target base
station of transmissions by the mobile terminal.
9. The method of claim 1, further characterized in that the
geometric information comprises one or more of: location
information for a source base station, location information for one
or more neighboring base stations associated with one or more
candidate cells that are candidates for selection as the handover
target; directional information relating the mobile terminal's
angle or direction to the source and/or neighboring base stations;
and timing information relating the mobile terminal's distance
relative to the source and/or neighboring base stations.
10. A base station configured to support mobility management in a
coordinated multipoint network, said base station including
transceiver circuits configured for transmitting signals to mobile
terminals and receiving signals from mobile terminals, and
inter-base station interface circuits configured for communicating
with one or more other base stations in the coordinated multipoint
network, and wherein said base station is characterized by one or
more processing circuits that are operatively associated with the
transceiver circuits and the inter-base station interface circuits
and configured to, when the base station is acting as a source base
station for a mobile terminal: determine geometric information for
the mobile terminal, based on signal measurements made for the
mobile terminal; and send the geometric information to a target
base station, for use in handover processing by the target base
station.
11. The base station of claim 10, further characterized in that the
one or more processing circuits comprise a handover controller
including a determining circuit configured to determine the
geometric information for the mobile terminal.
12. The base station of claim 11, further characterized in that the
handover controller further includes a handover target evaluation
circuit configured for identifying the target base station as the
handover target from among a number of candidate base stations that
are prospective handover targets.
13. The base station of claim 10, further characterized in that the
one or more processing circuits are configured to determine the
geometric information as one or more of: directional information
for the mobile terminal relative to the source base station; timing
advance information for the mobile terminal; round-trip-time
information for the mobile terminal; and location information for
the source base station.
14. The base station of claim 10, further characterized in that the
one or more processing circuits are further configured to: send the
geometric information to a number of candidate base stations that
are prospective handover targets for the mobile terminal; receive
measurement information from the candidate base stations
corresponding to signal measurements made by the candidate base
stations based at least in part on the geometric information sent
to them; and identifying the target base station as a preferred one
of the candidate base stations, based at least in part on
evaluating the measurement information received from the candidate
base stations.
15. A base station configured to support mobility management in a
coordinated multipoint network, said base station comprising
transceiver circuits configured for transmitting signals to mobile
terminals and receiving signals from mobile terminals, and
inter-base station interface circuits configured for communicating
with one or more other base stations in the coordinated multipoint
network, and wherein said base station is characterized by one or
more processing circuits that are operatively associated with the
transceiver circuits and the inter-base station interface circuits
and configured to, when the base station is acting as a prospective
target base station for a mobile terminal: receive geometric
information from a source base station for the mobile terminal,
said geometric information based on signal measurements made for
the mobile terminal; make signal measurements for the mobile
terminal, based at least in part on the geometric information; and
return the signal measurements to the source base station, for use
by the source base station in selecting a target base station for
handover of the mobile terminal.
16. The base station of claim 15, further characterized in that one
or more processing circuits are configured to use the geometric
information in improving the base station's reception of signals
from the mobile terminal, for making said signal measurements.
17. The base station of claim 15, further characterized in that the
geometric information comprises one or more of: directional
information for the mobile terminal relative to the source base
station; timing advance information for the mobile terminal;
round-trip-time information for the mobile terminal; and location
information for the source base station.
18. The base station of claim 17, further characterized in that
said one or more processing circuits are further configured to
derive directional information for the mobile terminal relative to
the base station based on the geometric information received from
the source base station.
19. The base station of claim 15, further characterized in that the
one or more processing circuits are configured to use the geometric
information received from the source base station to improve
transmissions by the base station to the mobile terminal.
20. The base station of claim 19, further characterized in that the
one or more processing circuits are configured to improve
transmissions by the base station to the mobile terminal by forming
or otherwise steering directional transmissions from the base
station to the mobile terminal, at least initially upon the mobile
terminal being handed over by the source base station to the base
station as the handover target for the mobile terminal.
21. The base station of claim 15, further characterized in that the
one or more processing circuits are configured to use the geometric
information to improve reception at the base station of
transmissions by the mobile terminal.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to wireless
communication networks, and particularly relates to mobility
management, e.g., handover-related operations, in a coordinated
multipoint network.
BACKGROUND
[0002] Coordinated Multi Point (CoMP) transmission and reception
refers to a system where the transmission and/or reception at
multiple, geographically separated antenna sites is dynamically
coordinated in order to improve system performance. The
coordination can either be distributed, by means of direct
communication between the different sites, or by means of a central
coordinating node.
[0003] CoMP is considered for LTE-Advanced as a tool to improve the
coverage of high data rates, the cell-edge throughput and/or to
increase system throughput. (LTE stands for 3G Long Term
Evolution.) In particular, the goal is to distribute the user
perceived performance more evenly in the network by taking control
of the inter-cell interference.
[0004] Downlink coordinated multi-point transmission implies
dynamic coordination among multiple geographically separated
transmission points. Examples of coordinated transmission schemes
include: coordinated scheduling and/or beamforming where data to a
single user equipment (UE) is instantaneously transmitted from one
of the transmission points, and scheduling decisions are
coordinated to control, e.g., the interference generated in a set
of coordinated cells. As another example, joint
processing/transmission may be used, where data to a single UE is
simultaneously transmitted from multiple transmission points, e.g.,
to (coherently or non-coherently) improve the received signal
quality and/or cancel actively interference for other UEs. Uplink
coordinated multi-point reception implies joint reception and
processing of signals at multiple, geographically separated points.
Scheduling decisions can be coordinated among cells to control
interference.
[0005] With the above in mind, a CoMP architecture can, in the case
of intra-eNB CoMP, be based on the Evolved NodeB (eNB or eNodeB)
acting as a central coordinating node. It is up to the vendor to
decide how to implement this, e.g., how many cells to control and
how to solve the communication between them and the eNB. Hence, the
interfaces can be non-standardized, which is more likely to allow
for more flexible and advanced CoMP methods to be used. From the
core network perspective intra-eNB CoMP is transparent, i.e., the
MME and the S-GW access the eNB over the S1 interface.
[0006] Inter-eNB CoMP could primarily be seen as distributed CoMP
(even though centralized operation also can be implemented). In
this case the coordination is carried out via direct communication
between the eNBs, e.g., over an (possibly) enhanced X2 interface.
From the core network perspective inter-eNB CoMP should be
transparent as well. The MME and the S-GW access the eNB over the
S1 interface. If necessary, control and data traffic is forwarded
over X2. FIGS. 1a and 1b illustrate example architectural
options.
[0007] As already mentioned above, downlink CoMP implies dynamic
coordination among multiple geographically separated transmission
points. At a high level, coordination schemes can be divided into
two categories: coordinated scheduling and/or beamforming; and
joint processing/transmission.
[0008] The first category is characterized in that data to a single
UE is instantaneously transmitted from one of the transmission
points, and in that scheduling decisions and/or generated beams are
coordinated in order to control the created interference. The main
advantages of these schemes compared to schemes involving joint
processing/transmission (see below) are that the requirements on
the coordination links and on the backhaul are much reduced, since
typically only information on scheduling decisions and/or generated
beams (and information needed for their generation) need to be
coordinated, and the user data does not need to be made available
at the coordinated transmission points, since there is only one
serving transmission point for one particular UE.
[0009] The second category, joint processing/transmission, is
characterized in that data to a single UE is simultaneously
transmitted from multiple transmission points, e.g., to (coherently
or non-coherently) improve the received signal quality and/or
actively cancel interference for other UEs. This category of
schemes puts higher requirements on the coordination links and the
backhaul since the user data needs to be made available at the
multiple coordinated transmission points. The amount of data to be
exchanged over the coordination links is also larger, e.g., channel
knowledge and computed transmission weights.
[0010] The most straightforward approach to CoMP is coordinated
scheduling, meaning that UEs are scheduled to be served by their
attached cells in a manner such that the mutual interference among
them (within the coordination set) is minimized, as illustrated in
FIG. 2. This can, to some extent, be seen as an extension of the
inter-cell interference coordination (ICIC) functionality present
already in LTE Rel-8. However, the previous ICIC schemes are based
on distinguishing users by geometry and the coordinated scheduler
assigns resource blocks to users such that SINR degradations due to
inter-cell interference are avoided as much as possible while
maintaining the baseline Reuse-1 resource usage. There was no
utilization of fast time-scale channel knowledge in order to
actively coordinate the involving cells.
[0011] In coordinated scheduling, each UE maintains the
conventional notion of correspondence with a serving cell. A UE
still transmits to and receives from only its attached (serving)
cell. However, the cell reselection may occur at a TTI interval for
best performance. With the knowledge of the path gain between all
active UEs and the cells and possibly additional information from
neighboring coordination sets, the controller can then carefully
re-order and select the UEs to be served in the same TTI or
resource block so that the interference experienced by each UE
meets a certain target. Since it is a scheduling based solution,
the physical layer transmission and reception of user data remain
unchanged. However, the CoMP architecture allows for proactive
control of the interference condition and more accurate link
adaptation, thereby leading to significant improvement (50-100%) in
cell edge user rate with either slight increase or decrease
(.+-.5%) in system throughput over LTE Rel-8 without any
coordination.
[0012] In the baseline coordinated scheduling, each coordination
set acts independently without communicating with neighboring
coordination sets. The controller in a coordination set determines
which UEs are to be served by the cells they are attached to in the
same TTI or resource block based on the path gain information and
factors such as transmission priority and packet size. In general,
each UE with an active session is placed in a queue according to
its transmission priority, which reflects the transmission need
based on some priority rule, e.g., how long it has stayed in the
queue. They are then placed one by one into the scheduling list if
a compatibility test is passed. The test criterion may be that the
SIR for all UEs in the schedule should be above a certain
threshold. It may also be that the highest interference for each UE
should not exceed a certain threshold. The target may be adjusted
according to traffic and buffer overflow conditions. The impact of
this baseline solution on standardization is limited if it is
considered as intra-eNB CoMP. All signaling functionalities are in
place in LTE Rel-8 already.
[0013] Coordinated beamforming, such as illustrated in FIG. 3, can
be used for downlink transmission if the transmission points in the
coordination set are equipped with antenna arrays. In this context,
coordinated beamforming is based on: transmit powers and
beamforming weights being jointly adapted for all UEs in the
coordination set, where beamforming weights are constant for all
frequencies (non-frequency dependent), and where one transmission
point serves multiple UEs (SDMA), as in multiuser MIMO.
[0014] A multi-antenna UE may receive multiple data streams. The
streams may be transmitted from multiple transmission points or a
single transmission point in the coordination set. In the second
case, the maximum number of data streams transmitted to a UE equals
the number of antenna clusters (sets of antenna arrays spaced far
enough apart) at the transmission point. Note that UE data needs to
be transported to multiple transmission points in the first case
which requires a fast high-capacity backhaul.
[0015] As with downlink CoMP, the most straightforward approach to
uplink CoMP is coordinated scheduling, i.e., the scheduling of UEs
to transmit are coordinated among the sites in such a way that the
interference among them is minimized. One such approach is
illustrated in FIG. 4. Thus, the same coordinated scheduling
approach as described for the downlink is also applicable for the
uplink; the only difference is how interference is calculated. All
required measurements and signaling functionality is already in
place in LTE Rel-8.
[0016] However, certain challenges arise in CoMP implementations,
particularly as relates to mobility management. In the context of
LTE, such challenges arise because existing mobility mechanisms in
LTE do not support macro-diversity transmission or reception, and
are therefore relying on fast and reliable indications for cell
change. Existing mechanisms in 3GPP Rel-8 include trigger criteria
and hysteresis values to support fast and accurate cell change in a
range of scenarios. However, the information available to the
network for target cell selection is mostly based on signal power
measurements, and does not accurately help to determine the most
suitable target cell for handover. In some cases, handover is
performed to unsuitable target cells, resulting either in failed
handovers or a return of the UE to the source cell after a short
while.
SUMMARY In one aspect, the present invention, in one or more
embodiments, improves mobility management--e.g., handover
processing enhancement--by providing/utilizing additional
information in combination with CoMP operation, for enhancing the
handover procedure. For example, the additional information is used
to improve judging the most suitable target cell. The additional
information can be, for example, angle of arrival or time advance
of the UE in handover, Round Trip Time (RTT), power headroom or
signal strength.
[0017] In at least one embodiment, the information used for
handover processing at source and/or target base stations is
enhanced or otherwise made richer by the use of "geometric
information" for mobile terminals. In one such embodiment, the
geometric information used for handover processing associated with
a given terminal indicates something about the mobile terminal's
position (relative or absolute).
[0018] In at least one embodiment, the information is used to
improve signal measurements made by one or more cells that are
candidates for receiving the mobile terminal in handover from a
source cell. For example, if the geometric information includes
directional information for the mobile terminal as regards the
source cell--e.g., angle-of-arrival information--a candidate base
station can derive the mobile terminal's approximate angle relative
to the candidate base station, and thus improve its
reception/measurement of signals from the mobile terminal. The
candidate base station thus returns better or more accurate signal
measurement information to the source base station, which therefore
can make an improved evaluation of the best candidate base station
to select for as the handover target.
[0019] Additionally, or alternatively, in one or more embodiments,
a target base station uses the geometric information to improve its
transmission to and/or reception from the mobile terminal. For
example, it may use the geometric information to better receive the
Physical Random Access Channel (PRACH) or other uplink signal
reception--e.g., Sounding Reference Signals (SRSs)--from the mobile
terminal, and/or to improve its downlink transmissions to the
mobile terminal. In that latter case, one or more embodiments use
the geometric information to bias or otherwise direct beamforming
toward the mobile terminal's position, or at least in the direction
of the mobile terminal. Such directing or steering of the target
base station's transmissions improves signal reception at the
mobile terminal and can reduce interference seen by other terminals
or elsewhere in the network. Such directing or steering can be done
at least initially upon or as part of receiving the mobile terminal
in handover, thereby improving handover reliability.
[0020] Of course, the present invention is not limited to the above
features and advantages. Indeed, those skilled in the art will
recognize additional features and advantages upon reading the
following detailed description, and upon viewing the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1a) and 1b) illustrate intra- and inter-eNB scenarios
in a Coordinated Multipoint (CoMP) Network.
[0022] FIG. 2 illustrates downlink CoMP, based on coordinated
scheduling.
[0023] FIG. 3 illustrates downlink CoMP, based on coordinated
beamforming.
[0024] FIG. 4 illustrates uplink CoMP, based on coordinated
scheduling.
[0025] FIG. 5 illustrates uplink CoMP, based on coordinated
beamforming.
[0026] FIG. 6A illustrates on embodiment of a method of mobility
management in a CoMP network, while FIG. 6B illustrates
corresponding details for the steps depicted in FIG. 6A.
[0027] FIG. 7 illustrates a simplified example wireless
communication network, e.g., a CoMP network, including base
stations configured for one or more embodiments of mobility
management enhancements as taught herein.
[0028] FIG. 8 illustrates example embodiments for a source base
station and a candidate base station that is a target for handover
of a mobile terminal from the source base station.
DETAILED DESCRIPTION
[0029] In one or more embodiments presented herein, when a UE is
evaluating cells for handover target candidates in a CoMP scenario,
the target cell selection considers the angle of the source cell in
order to determine the angle from the target cell, so that the
target cell can direct a beam towards the UE in handover. This
action, which is depicted in FIG. 5, results in a concentration of
energy to and from the UE from the target cell, and therefore
contributes to a more reliable handover. In addition, the target
cell selection can also make use of time advance information from
the source and/or the target cell to determine a more accurate
position of the UE. As noted, the angle/direction and timing
information all can be considered individually or collectively to
be examples of the geometric information used herein to improve
mobility management.
[0030] In the same or other embodiments, the source cell evaluates
the quality of the target cell by ordering the target cell to
measure on the UE and to report the measured quality back. The
quality measure can be e.g., the received signal quality of the
uplink reference signals, the quality of the received Physical
Uplink Control Channel (PUCCH) or the quality of received SRS. The
coordination between the cells can, e.g., be done using the X2
interface or a proprietary interface between the source eNodeB and
the target eNodeB(s).
[0031] Information provided to the target cell for performing the
measurement may include angle of direction, time advance, RTT,
power headroom, signal strength or geographical coordinates of the
eNB from the source cell. Again, individual elements of such
information and/or the collection of all or some of this
information, is referred to herein as "geometric information".
[0032] The source cell may evaluate the quality of multiple cells
that are candidates for receiving the UE in handover, either
sequentially or in parallel, using the mechanism described above.
Thus, after receiving the measurement information from one or
multiple target cells, the source cell decides on the target cell
for handover and initiates the handover to the target cell.
[0033] In the same or other embodiments, the target cell uses the
measured angle and/or time advance for reception of PRACH during
handover, in order to improve the quality of the received signal in
the eNodeB.
[0034] Further, in at least one embodiment, the target cell uses
the measured angle and/or time advance for transmission of downlink
data after handover, in order to improve the quality of the
received signal in the UE, and to avoid excess interference.
[0035] Of course, those skilled in the art will appreciate that,
while the above teachings are advantageously applied in LTE and LTE
Advanced systems, they are not limited to those applications. For
example, the above teachings may be advantageously applied in other
types of networks, such as other Orthogonal Frequency Division
Multiplexing (OFDM) based systems, including WiMax.
[0036] In that broader context, the invention proposed herein
provides mechanisms (e.g., signaling and processing operations) for
enhancing handover performance in conjunction with the introduction
of CoMP. The usage of angle and/or time advance information, or
other such geometric information, for selecting the most suitable
target cell before initiating the handover leads to more reliable
target cell selections and thus provides more reliable handover
decisions. In addition, excess interference can be avoided, and
UE/base station transmissions made more reliable and robust, by
properly directing energy to the UE from the target cell.
[0037] With the above non-limiting advantages in mind, one
embodiment of the invention proposed herein comprises a method of
mobility management in a coordinated multipoint network. FIG. 6A
illustrates an example method 100, comprising sharing geometric
information for a mobile terminal between two or more cells in the
coordinated multipoint network (Step 102), where the geometric
information is determined from signal measurements made for the
mobile terminal. Further, the method 100 includes determining a
handover target for the mobile terminal from among the two or more
cells, based at least in part on the geometric information (Step
104). For example, the source (serving) base station in the source
cell shares geometric information with one or more neighboring base
stations that control one or more cells that are candidates for
selection as the handover target.
[0038] As shown by way of example in FIG. 6B, in one or more
embodiments, the step 102 of sharing the geometric information
comprises a source cell sending the geometric information to a
number of candidate cells that are prospective handover targets for
the mobile terminal (Step 102A), and receiving measurement
information from the candidate cells (Step 102B). The received
measurement information corresponds to signal measurements made by
the candidate cells based at least in part on the geometric
information sent to them by the source cell. In this context, the
step 104 of determining the handover target comprises identifying
the handover target as a preferred one of the candidate cells,
based at least in part on evaluating the measurement information
received from the candidate cells. (Such evaluation can be
performed by the source cell's controlling base station. Note that
"base station" as used herein should be broadly construed to
encompass, by way of non-limiting example, eNBs from LTE/LTE
Advanced, as well as WiMax base stations or access points.)
[0039] In at least one embodiment, the geometric information
relates to a relative or absolute position of the mobile terminal,
with respect to one or more base stations in the coordinated
multipoint network. In particular, in one or more embodiments, the
geometric information includes one or more of: directional
information for the mobile terminal relative to a source base
station; timing advance information for the mobile terminal; and
round-trip-time information for the mobile terminal.
[0040] Also, as noted, the step of sharing the geometric
information may comprise a source base station in the coordinated
multipoint network sending the geometric information to a candidate
base station in the coordinated multipoint network that is a
candidate for being selected as the handover target. In association
with this, the contemplated method further includes the candidate
base station using the geometric information to improve signal
measurements made by the candidate base station for the mobile
terminal, wherein said signal measurements are evaluated in said
step of determining the handover target.
[0041] As such, in one or more embodiments, the geometric
information is used in a target base station associated with a
candidate cell in the coordinated multipoint network that is
identified as the handover target, to improve transmissions by the
target base station to the mobile terminal. Further, the target
base station may use the geometric information to form or otherwise
steer directional transmissions from the target base station to the
mobile terminal, at least initially upon the mobile terminal being
handed over to the target base station.
[0042] As one example, using the geometric information comprises
using direction information present in the geometric information as
received from the source base station, to direct transmissions
toward the mobile terminal, or deriving directional information
from the received geometric information--e.g., the target base
station can use directional information and/or transmission timing
information relating the terminal to the source base station, to
estimate the terminal's direction or angle relative to the target
base station.
[0043] In any case, in one or more embodiments, the target base
station may use the geometric information to improve reception at
the target base station of transmissions by the mobile terminal.
Further, as noted, the geometric information may comprise one or
more of: location information for a source base station, location
information for one or more neighboring base stations associated
with one or more candidate cells that are candidates for selection
as the handover target; directional information relating the mobile
terminal's angle or direction to the source and/or neighboring base
stations; and timing information relating the mobile terminal's
distance relative to the source and/or neighboring base
stations.
[0044] As an example of a network and associated base stations,
FIG. 7 illustrates a wireless communication network 10, which is
configured as a CoMP network in one or more embodiments. As a
non-limiting example, the network 10 comprises an LTE or LTE
Advanced network configured for CoMP operation.
[0045] One sees that the network 10 includes a number of base
stations 12, which are depicted for clarity as 12-1, 12-2, and so
on. These numbering suffixes are used when convenient for
distinguishing operations at one base station 12 from another base
station 12, and when such distinctions are not needed the reference
numeral "12" serves a generic identifier for a given base station.
Those skilled in the art will appreciate that where the network 10
comprises an LTE or LTE Advanced network, the base stations 12 are
eNBs.
[0046] Each base station 12 defines or otherwise controls a cell
14, wherein the cells are referred to as 14-1, 14-2, and so on,
when necessary to distinguish one cell from another, and otherwise
the reference number "14" refers to any given cell in the singular
or given cells in the plural. Those skilled in the art will also
appreciate that cells may overlap, may be sectorized, may be
uniform or non-uniform in size, and subject to other variations. In
that sense, the figure should be understood as a non-limiting
example. It should also be understood that the network 10 may
include other nodes or entities, such as those in a Core Network
(CN), which is not shown for the sake of simplicity.
[0047] One also sees that the base stations 12 may be
communicatively interlinked by an inter-base station interface 16,
such as the "X2" interface used in LTE. (Only one inter-base
station communication link is shown for simplicity.) Such
inter-base station linking can be advantageously used for sharing
information between base stations, including the geometric
information contemplated herein, for improving mobility management.
For example, the geometric information is used to improve handover
target selection and/or to improve transmission and/or reception
between the mobile terminal in the target cell.
[0048] Referring now to FIG. 8, one sees a base station 12-1 and a
base station 12-2. Both base stations 12 may be configured
identically, or there may be differences between them. In any case,
for the purposes of FIG. 8 and the following discussion, one may
assume that the base station 12-1 is the source base station for a
given mobile terminal 20, and that the base station 12-2 is a
candidate base station for handover target selection and/or that
the base station 12-2 has been selected as the handover target.
[0049] Referring to either one now, the base station 12 is
configured to support mobility management in a coordinated
multipoint network 10, and it comprises transceiver circuits 40
configured for transmitting signals to mobile terminals 20 and
receiving signals from mobile terminals 20, and inter-base station
interface circuits 42 configured for communicating with one or more
other base stations in the coordinated multipoint network 10. The
base station 12 further includes one or more processing circuits 44
operatively associated with the transceiver circuits 40 and the
inter-base station interface circuits 42.
[0050] These processing circuits 44 are configured to, when the
base station 12 is acting as source base station for a mobile
terminal 20 (e.g., acting as base station 12-1): determine
geometric information for the mobile terminal 20, based on signal
measurements made for the mobile terminal 20, and send the
geometric information to a target base station (e.g., base station
12-2), for use in handover processing by the target base
station.
[0051] Referring specifically to the base station 12-1 now for
source base station processing details, the one or more processing
circuits 44 comprise a handover controller 50-1 including a
determining circuit 52 configured to determine the geometric
information for the mobile terminal 20. In one or more embodiments,
the handover controller 50-1 further includes a handover target
evaluation circuit 54 configured for identifying the target base
station as the handover target from among a number of candidate
base stations that are prospective handover targets. For example,
base station 12-1 may evaluate base stations 12-2, 12-3, and so on,
as candidates for receiving the mobile terminal 20 in handover.
[0052] In at least one such embodiment, the one or more processing
circuits 44 of the base station 12-1 are configured to determine
the geometric information as one or more of: directional
information for the mobile terminal 20 relative to the source base
station 12-1; timing advance information for the mobile terminal
20; round-trip-time information for the mobile terminal 20; and
location information for the source base station 12-1.
[0053] Further, in at least one such embodiment, the one or more
processing circuits 44 of the base station 12-1 are further
configured to send the geometric information to a number of
candidate base stations that are prospective handover targets for
the mobile terminal 20, e.g., to base stations 12-2, 12-3, and so
on, and to receive measurement information from the candidate base
stations corresponding to signal measurements made by the candidate
base stations based at least in part on the geometric information
sent to them. Further, the processing circuits 44 of the base
station 12-1 are configured to identify the target base station as
a preferred one of the candidate base stations, based at least in
part on evaluating the measurement information received from the
candidate base stations.
[0054] Now referring to the base station 12-2 as a candidate base
station and/or as a selected target base station, the base station
12-2 is configured to support mobility management in a coordinated
multipoint network 10 and it comprises, as noted, transceiver
circuits 40 configured for transmitting signals to mobile terminals
20 and receiving signals from mobile terminals 20. The base station
12-2 further comprises inter-base station interface circuits 42
configured for communicating with one or more other base stations
in the coordinated multipoint network 10, and one or more
processing circuits 44 operatively associated with the transceiver
circuits 40 and the inter-base station interface circuits 42.
[0055] These processing circuits 44 are configured to, when the
base station 12-2 is acting as a prospective target base station
for a mobile terminal 20, receive geometric information from a
source base station 12-1 for the mobile terminal 20, where the
geometric information is based on signal measurements made for the
mobile terminal 20--e.g., measurements made by or for the source
base station 12-1. In complementary fashion, the one or more
processing circuits 44 of the prospective target base station 12-2
are configured to make signal measurements for the mobile terminal
20, based at least in part on the geometric information, and return
the signal measurements to the source base station 12-1, for use by
the source base station 12-1 in selecting a target base station
(from among the candidates) for handover of the mobile terminal
20.
[0056] In one or more embodiments, the one or more processing
circuits 44 of the base station 12-2 are configured to use the
geometric information in improving the base station's reception of
signals from the mobile terminal 20, for making said signal
measurements. That is, the signal measurements made by the base
station 12-2 can be improved through use of the geometric
information, meaning that such measurement information provides a
better basis for selecting the handover target.
[0057] In one or more embodiments, the geometric information
comprises one or more of: directional information for the mobile
terminal 20 relative to the source base station 12-1; timing
advance information for the mobile terminal 20; round-trip-time
information for the mobile terminal 20; and location information
for the source base station 12-1.
[0058] In the same or other embodiments, the one or more processing
circuits 44 of the base station 12-2 are further configured to
derive directional information for the mobile terminal 20 relative
to the base station 12-2 based on the geometric information
received from the source base station 12-1.
[0059] Still further, in the same or other embodiments, the one or
more processing circuits 44 of the base station 12-2 are configured
to use the geometric information received from the source base
station 12-1, to improve transmissions by the base station 12-2 to
the mobile terminal 20. In at least one such embodiment, such
processing circuits 44 are configured to improve transmissions by
the base station 12-2 to the mobile terminal 20, by forming or
otherwise steering directional transmissions from the base station
12-2 to the mobile terminal 20, at least initially upon the mobile
terminal 20 being handed over by the source base station 12-1 to
the base station 12-2, as the handover target for the mobile
terminal 20.
[0060] In a similar fashion, in one or more embodiments, the one or
more processing circuits 44 of the base station 12-2 are configured
to use the geometric information to improve reception at the base
station 12-2 of transmissions by the mobile terminal.
[0061] With the above examples in mind, those skilled in the art
will appreciate that all base stations 12 may have like or similar
hardware and/or software features and operations, and that the
behavior and processing of any given one of the base stations 12
can be source base station or prospective target (or target) base
station, depending upon the role it is playing at any given time,
with respect to any given mobile terminal 20. Thus, a given base
station 12 may, at a given time and with respect to a given mobile
terminal 20, configure itself as the depicted base station 12-1,
while at other times and/or with respect to other mobile terminals,
it may configure itself as the depicted base station 12-2.
[0062] On this point, those skilled in the art will appreciate that
the depicted processing circuits 44 can be dedicated hardware
circuits, programmable circuits, or some mix of the two. In at
least one embodiment, each base station 12 includes memory, a disk,
or another computer-readable medium storing one or more computer
programs comprising program instructions. Execution of such stored
program instructions configure the one or more processing circuits
44 (in either or both base stations 12-1 and 12-2) in accordance
with the above-described mobility management operations, and it
will be therefore understood that the such processing circuits 44
can include one or more than one microprocessor-based circuit, or
other programmable digital processor.
[0063] Of course, those skilled in the art will recognize that the
present invention is not limited by the foregoing description or by
the accompanying drawings.
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